The present invention relates to retroreflective sheeting employing microprism formations to retroreflect the light rays impinging thereon, and more particularly, to a method for producing such retroreflective sheet material exhibiting a high degree of whiteness and retroreflectivity, and to the sheet materials produced thereby.
Retroreflective sheet material is widely employed for a variety of safety and decorative purposes, and is particularly useful when night time visibility is significant under conditions of low ambient light. In retroreflective materials, the light rays impinging upon the front surface are reflected back towards the source of the illumination in a substantially parallel path. In situations where headlights or search lights, boats and aircraft are the only source of illumination, this ability to retroreflect the bulk of the rays falling thereon is especially significant.
Minnesota Mining and Manufacturing Corporation has manufactured retroreflective sheeting utilizing minute glass beads embedded in a matrix of synthetic resin to provide such retroreflection, and these materials have been sold under the trademark SCOTCHLITE. Illustrative of such materials is Bergeson et al U.S. Pat. No. 4,637,950 granted Jan. 20, 1987.
Applicant's assignee, Reflexite Corporation, has been marketing under the trademark REFLEXITE, reflective sheeting employing microprisms formations to produce such retroreflection. Illustrative of such materials is Rowland U.S. Pat. No. 3,689,346 granted Sept. 5, 1972.
Among the applications for such retroreflective materials are reflective tapes and patches for clothing of firemen, reflective vests and belts, bands for posts and barrels, traffic cone collars, highway signs, warning reflectors, and the like.
It is known that a cube corner prism surrounded by air will retroreflect light incident upon the front surface or base thereof impinging upon and the prism surfaces. However, it is common to coat the prism surfaces with a reflective or specular material such a vacuum deposited aluminum layer to provide a retroreflective interface. In this manner, adhesive and other backing materials may be deposited around the cube corner prisms for lamination to backing sheets and other structures.
Moreover, when using microprisms of less than about 0.010 inch on center spacing, it has been found that there is less retroreflection of light incident at an angle of 30.degree. or more if the surfaces of the microprisms are unmetallized. Thus, when making such microprism sheeting, it is desirable to have some portion of the microprisms coated with a specular material which will have the tendency to reflect light impinging at higher angles of incidence.
The use of a metallized aluminum coating on the prism surfaces tends to produce a grey coloration to the observer in ambient light or daylight conditions. In some applications, this grey appearance is considered aesthetically undesirable.
To prevent or minimize the failure of such retroreflective sheet materials incident to penetration of salt or other corrosive materials, or moisture, about the retroreflective formations, it is common to provide a protective backing over the formations and to seal that backing to the retroreflective sheeting in a grid pattern to produce discrete cells containing isolated clusters of retroreflective formations. Illustrative of such grid structures is Bergeson et al U.S. Pat. No. 4,637,950 granted Jan. 20, 1987.
It is an object of the present invention to provide a novel sealed retroreflective sheeting using microprism formations overlaid by a protective backing element and which exhibits high reflectivity and a desirable degree of whiteness to the observer in daylight.
It is also an object of the present invention to provide such a retroreflective sheet material which may be readily fabricated and which is durable and resistant to the elements. Another object of the present invention is to provide novel methods for fabricating such retroreflective sheet material which are relatively simple and relatively economical, and which produce long-lived materials.